ALTERATION OF THE EARLIER ANDESITE. 211 



alteration has been often complete, yet there has been no very great increase 

 or decrease in the original elements. The original combinations of these elements 

 have been broken up, and hydrated silicates, with abundant carbonates and sulphides, 

 have formed, indicating only the presence of carbonic acid and hydrogen sulphide 

 in the altering waters. Since the quartz-sericite alteration of the earlier andesite 

 grades into the chlorite-calcite alteration by all possible stages, it is probable 

 that both were produced at the same time and by the same waters; and since 

 the transition from the quartz-sericite alteration to the metalliferous quartz 

 veins is similarly perfect, the waters are clearly those which have produced the 

 mineralization. Within the main circulation channels, therefore, these waters 

 introduced silica, potash, and the metallic sulphides, and abstracted other materials. 

 As they penetrated the rock away from these channels they ceased to deposit 

 metals, except possibly in trifling quantity," while the excess of silica and potash 

 was still deposited, failing with increasing distance. Finally, the changes in the 

 calcite-cblorite alteration show that only the common gases above mentioned, so 

 commonly present in surface hot springs, were left in the mineralizing waters, 

 which therefore had little to precipitate and small power to abstract. 



The successive precipitation so plainly demonstrated probably took place by 

 reactions with the wall rock, which therefore acted as a screen for the traversing 

 solutions. 



REFRACTORINESS OF POTASH FELDSPARS. 



In arguing that the formation of potash minerals in the veins and in the wall 

 rock shows a relative excess of potash in the mineralizing waters, it must be taken 

 into consideration that potash feldspars are ordinarily more refractor}' to altering 

 waters than the soda-lime varieties. Comparison of analyses of fresh rocks and 

 of rocks altered by surface weathering usually show that the loss of soda is 

 greater than that of potash.* It is also true, as pointed out by Lindgren,"" that 

 one of the most prominent minerals formed by metasomatic processes in and 

 near veins is a potassium mica, such as muscovite, and that the most prominent 

 process brought about by the waters is the progressive increase of potash and 

 the decrease of soda. At the Boulder Hot Springs, described by Weed, rf sericite 

 and in one case adularia had been deposited from the waters, which contain 

 chiefly sodium sulphate, carbonate arid chloride, calcium carbonate, and silica; 

 no potassium is recorded. Near the Comstock lode, potash, as compared with 

 soda, is more important in the altered than in the fresh rock/ showing that 



"Sampling of the Mizpah mine, under the direction of Mr. John Hays Hammond, showed that the earlier andesite 

 forming the walls of the vein runs in values from $0.50 to $2 a ton, as compared with many times that value in the vein. 

 6 Merrill, G. M., Rocks, Rock-weatliering, and Soils, p. 236. 

 eLindgren, W., Trans. Am. Inst. Min. Eng., vol. 30, p. 690. 

 dWeed, W. H., Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2, p. 246. 

 Lindgren, W., Trans. Am. Inst. Min. Eng., vol. 30. p. 647. 



